The present invention relates generally to the fabrication of three-dimensional objects using extrusion-based layered manufacturing techniques. Specifically, it relates to modeling filament used as a feedstock in a fused deposition three-dimensional modeling machine.
Three-dimensional models are used for functions including aesthetic judgments, proofing a mathematical model, forming hard tooling, studying interference and space allocation, and testing functionality. Extrusion-based layered manufacturing machines build up three-dimensional models by extruding solidifiable modeling material from a nozzle tip carried by an extrusion head onto a base. The modeling material flows when heated, solidifies upon a drop in temperature, and adheres to the previous layer with an adequate bond upon solidification. Suitable materials include waxes, thermoplastic resins, and various metals. Movement of the extrusion head with respect to the base is performed in a predetermined pattern under computer control, in accordance with design data provided from a computer aided design (CAD) system.
Examples of extrusion-based apparatus and methods for making three-dimensional objects are described in Valavaara U.S. Pat. No. 4,749,347; Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433, Crump et al. U.S. Pat. No. 5,503,785; Danforth et al. U.S. Pat. No.5,738,817, Danforth, et al. U.S. Pat. No. 5,900,207; Batchelder et al. U.S. Pat. No. 5,764,521 and Dahlin et al. U.S. Pat. No. 6,022,207, Swanson U.S. Pat. No. 6,004,124, Stuffle et al. U.S. Pat. No. 6,067,480 and Batchelder, et al. U.S. Pat. No. 6,085,957, all of which are assigned to Stratasys, Inc., the assignee of the present invention.
Modeling material may be provided to the extrusion head of a layered deposition modeling machine in various forms, including a liquid or solid feedstock of such material. The extrusion head will heat a solid feedstock to a flowable temperature for deposition. One technique provides the modeling material to the extrusion head in the form of a filament strand. A pressurization means is used to extrude molten modeling material from the extrusion head.
Stratasys® FDM® three-dimensional modeling machines of the current art use a software program to “slice” the CAD design of an object into multiple horizontal layers. The machines extrude modeling material in fluent strands, termed “roads” thereby building up the object layer-by-layer. Each extruded road has a thickness equal to the height of a slice. The material being extruded fuses to previously deposited material and solidifies upon a drop in temperature to form a three-dimensional object resembling the CAD model. The modeling material is typically a thermoplastic or wax material.
In the most frequently implemented configuration, the material feedstock is in the form of a filament. In the Stratasys® FDM® modeling machines of the current art which use a filament feedstock, modeling material is loaded into the machine as a flexible filament wound on a supply reel, such as disclosed in U.S. Pat. No. 5,121,329. Typically, the filament has a small diameter, such as on the order of 0.070 inches. The extrusion head, which includes a liquifier and a dispensing nozzle, receives the filament, melts the filament in the liquifier, and extrudes molten modeling material from the nozzle. Motor-driven feed rollers advance the strand of the filament into the liquifier. The motor-driven feed rollers push filament into the liquifier to create a “liquifier pump”, wherein the filament itself serves as the piston. As the feed rollers advance filament into the liquifier, the force of the incoming filament strand extrudes the flowable material out from the nozzle. Typical extruded flow rates of the current art range from 0 to 20,000 micro-cubic inches per second.
The volumetric flow rate of the material extruded from the nozzle is a function of the rate at which the filament is advanced to the head. The flow rate is thus commanded by controlling the speed of advancement of filament into the liquifier. Optimally, the liquifier should reproduce the motion of the rollers in the emerging flow volume with perfect fidelity. A controller controls the movement of the extrusion head in a horizontal x-y plane, controls the movement of the base in a vertical z-direction, and controls the rate at which the feed rollers advance filament. By controlling these processing variables in synchrony, the modeling material is deposited in “roads” layer-by-layer along tool paths defined from the CAD model. The material being extruded fuses to previously deposited material and solidifies to form a three-dimensional object resembling the CAD model.
The extruded roads have a cross-sectional area that should ideally be precisely controlled to create an accurate model. Usually, a constant bead width is desired. The bead width is related to the extrusion head velocity, as well as the size of the nozzle orifice and the flow rate of material from the nozzle. The bead width is also affected by the clearance between the extruding nozzle tip and a previously extruded layer (or the base). When the extrusion head velocity changes, the output flow rate must change accordingly or a constant bead width will not be attained.
In an effort to achieve predictable extruded flow rates, modeling filament is manufactured to meet tolerance limits. Filament in the form of a cylinder with a target diameter of 0.070 inches in diameter is manufactured with a tolerance of +/−0.0025 inches. The filament is manufactured using an extrusion process wherein the target diameter is sought. The prior art methods verify tolerance using a laser micrometer during the manufacturing process and by measuring spooled filament with a micrometer after the filament has dried. The filament is deemed satisfactory it if there are no measurements larger than 0.0725 inches or smaller than 0.0675 inches.
In the modeling systems of the current art, bead width errors arise, particularly at start points and end points of the tool path, for instance, at the location of a “seam” (i.e., the start and end point of a closed-loop tool path). Bead width errors arise in part due to inconsistent flow rates. These errors cause undesired inconsistencies in the shape of the resulting model.
A technique for controlling the output flow rate so as to minimize bead width and seam errors is disclosed in U.S. Pat. No. 6,085,957. The '957 patent teaches the instantaneous sensing of filament diameter of a filament strand as it enters the liquifier, and consequent instantaneous adjustment of the feed rate of filament into the liquifier. While this technique does reduce errors, its has disadvantages in that each modeling machine must be equipped with a sensor, adding to the cost and maintenance of the machines. Also, using this technique, some errors have been observed to remain.
It would be desirable to further minimize bead width and seam errors in models made using fused deposition modeling techniques.